Molten salt synthesis of titanate pyrochlore waste-forms

Actinide chlorides, such as those arising from pyrochemical reprocessing operations can be problematic to immobilise, as the high chloride content often makes their solubilities in melts very low, and even in small quantities can seriously affect the properties of the waste-form. Rather than attempt to immobilise the chlorine, one potential approach is to utilise the chloride salt as a reaction medium from which the An(III) cations can be extracted and immobilised. To this end, lanthanide titanate pyrochlores have been prepared by molten salt synthesis in CaCl2:MgCl2, CaCl2:NaCl and MgCl2:NaCl eutectics. Single-phase pyrochlore is found to be formed at temperatures as low as 650 °C in the CaCl2:NaCl system, whereas in the CaCl2:MgCl2 and MgCl2:NaCl eutectics reaction with Mg produces a magnesium titanate secondary phase. Compositions of Yb2Ti2O7 doped with Sm3+ as an actinide surrogate have been synthesised, and cold-pressing and sintering at 1500 °C yields fully dense pellets.